The size and thickness of portable wireless communication apparatuses, such as mobile phones, have been rapidly reduced. In addition, the portable wireless communication apparatuses have been transformed from apparatuses to be used only as conventional telephones, to data terminals for transmitting and receiving electronic mails and for browsing web pages of WWW (World Wide Web), etc. Further, since the amount of information to be handled has increased from that of conventional audio and text information to that of pictures and videos, a further improvement in communication quality is required. In addition, the portable wireless communication apparatuses are required to handle various applications for voice calls as telephones, data communication for browsing web pages, viewing of television broadcasts, etc. In such circumstances, it is necessary to provide an antenna apparatus operable in a wide frequency range in order to perform wireless communications for the respective applications.
According to the prior art, there were antenna apparatuses covering a wide frequency band and adjusting the resonance frequency, including, for example, an antenna apparatus adjusting the resonance frequency by providing an antenna element portion with a slit as described in Patent Literature 1, and a notch antenna having a trap circuit at a slit as described in Patent Literature 2.
The antenna apparatus of Patent Literature 1 is configured to include: a planar radiating element (radiating plate); a ground plate opposing thereto in parallel; a feed section located nearly at the center of an edge of the radiating plate and supplying a high-frequency signal; a short-circuiting section shorts-circuiting the radiating plate and the ground plate near the feed section; and two resonators formed by providing a slit to an edge of the radiating plate nearly opposing to the feed section. The degree of coupling between the two resonators is optimized by adjusting the shape and dimensions of this slit, or by loading a reactance element or a conductor plate on the slit. Thus, it is possible to obtain a small and low-profile antenna with suitable characteristics.
According to the notch antenna of Patent Literature 2, the slit can be open for radio frequency signals at the position of the trap circuit when the antenna is to resonate in a low-frequency communication band, and the slit can be closed for radio frequency signals at the position of the trap circuit when the antenna is to resonate in a high-frequency communication band. Thus, it is possible to change the resonant length of the notch antenna in an appropriate manner according to a frequency communication band in which the antenna is to resonate.
In addition, an antenna apparatus of Patent Literature 3 is configured to include: a substrate; a plurality of antenna elements located on the substrate and fabricated in a planar shape; and at least one isolation element located on the substrate between the plurality of antenna elements and grounded to a ground portion. The isolation element fabricated between the antenna elements is used to prevent mutual interference between the antenna elements, thus advantageously preventing distortion of a radiation pattern. In addition, the isolation element can operate as a parasitic antenna by connecting the isolation element to a ground plane, thus advantageously increasing the output gain. In addition, the isolation element and the antenna elements can be fabricated by only etching metal films stacked on the substrate in a predetermined configuration. Therefore, the fabrication is facilitated, e.g., a metal film on the substrate forms the isolation element, thus capable of fabricating an antenna apparatus of a planar structure substantially close to two dimensions.